Objective Diagnosis of the Eyewall Replacement Cycle in Typhoon Sinlaku

Typhoon Sinlaku underwent an eyewall replacement cycle shortly before making landfall over Taiwan in September 2008 during the Tropical Cyclone Structure – 2008 (TCS-08) field program. Eyewall replacement cycles in tropical cyclones are often associated with intensity fluctuations and increased predictability challenges. Currently, microwave satellite imagery is an accepted and often effective means by which to detect various stages in the eyewall replacement cycle; however, the near 12-h temporal resolution and hit-or-miss spatial sampling of microwave sensors are insufficient to capture the detailed evolution of the eyewall replacement cycle process, which may last less than 24 h.

Here, the eyewall replacement cycle during Typhoon Sinlaku is examined using satellite and aircraft observations. Differences in water vapor and infrared satellite brightness temperatures from geostationary sensors are used to detect deep convection that penetrates the tropopause. The geostationary data are transformed into a cylindrical grid about the storm center, and the water vapor – infrared differences are examined over varying radial bands. A new diagnostic technique is then used to map this deep convection over time. In the case of Typhoon Sinlaku, the deep convection is mapped during a 72-h period that includes the eyewall replacement cycle and provides clear evidence of inner eyewall decay as well as outer eyewall development and subsequent contraction. These results are compared with other satellite data and TCS-08 aircraft observations, and links between the evolution of deep convection and changes in intensity are assessed.

The eyewall mapping technique is then extended to other cases to examine the potential for improving intensity forecast accuracy during eyewall replacement cycles. While diagnostic in nature, this eyewall mapping technique may be applied on a real-time basis and provide the forecaster with unique, objective insight regarding the evolution of a tropical cyclone throughout its lifecycle and be of particular use in the diagnosis of eyewall replacement cycles. The technique may also provide new insights into the behavior of deep convection and possible correlation to periods of rapid intensification and rapid decay.